Shepherd is claiming that Sally helped himself to $20K from their joint bank account before he abandoned her… and the money could end up as a key piece of evidence in their custody war.

Sherri, according to sources close to her, say she was in the dark until she looked at a recent bank statement and saw the money was gone.

Shepherd wants a judge to invalidate the contract she and Lamar Sally signed with a surrogate on grounds Lamar defrauded her — that the only reason he wanted the baby was so he could divorce her and collect child support.

Reportedly, Sherri will use the $20K withdrawal as proof Lamar is motivated purely by money.

Lamar says, he used the money for baby expenses, including trips he took to Pennsylvania to be with the surrogate. He also says, Shepherd was aware that he took the money out of their joint account.